Electrostatic printing



July 25, 1961 M. L. SUGARMAN, JR 2,993,737

ELECTROSTATIC PRINTING Filed Aug. 14, 1956 IN VEN TOR.

MEYER L. SUEARMAN1JR.

i7 7' OKIYE y United States Patent 2,993,787 ELECTROSTATIC PRINTING Meyer L. Sugarman, Jr., Princeton, N.J., assignor to Radio Corporation of America, a corporation of Delaware Filed Aug. 14, 1956, Ser. No. 604,046 2 Claims. (Cl. 961) This invention relates to electrostatic printing and particularly, but not necessarily exclusively, to improved compositions and improved methods for producing recording elements especially adapted for use in electrostatic printing processes.

An electrostatic printing process is that type of process for producing a visible record, reproduction or copy which includes an an intermediate step, converting a light image or electrical signal into an electrostatic charge pattern on an electrically-insulating layer. The process may also include the conversion of the charge pattern into a visible image which may be a substantially faithful reproduction of an original except that it may be a different size, color or contrast range.

A typical electrostatic printing process may include preparing an electrophotographic recording element, for example, by coating a surface of a backing with a photoconducting insulating material such as selenium, anthracene, or zinc oxide dispersed in an electrically-insulating, film-forming, water insoluble vehicle such as a silicone resin. An over-all'electrostatic charge is produced on the surface of the photoconducting material and a light image is focused on the charged surface, discharging the portions irradiated by the light rays, while leaving the remainder of the surface in a charged condition thereby forming an electrostatic image. The electrostatic image is rendered visible by applying a developer powder which is held electrostatically to the charged areas of the sheet. The powder image thus formed may be fixed directly to the photoconductive coating or it may be transferred to another surface upon which the reproduced image may be desired and then fixed thereon.

According to a first prior art process, the recording element for electrostatic printing is prepared by first mixing the finely-divided photoconductor and the electricallyinsulating, film-forming, water-insoluble vehicle with a volatile organic solvent for the vehicle, for example, by mixing zinc oxide and a silicone resin with toluene. This mixture is coated on the backing member by any standard coating procedure, dried and then cut or punched to a desired size.

According to a second prior art process, a recording element for electrostatic printing is prepared by mixing a finely-divided photoconductor with an aqueous dispersion of an electrically-insulating, film-forming, water insoluble vehicle, for example, by mixing zinc oxide with polyvinyl acetate resin dispersed in water. This mixture is coated on a backing member by any standard coating procedure, dried and then cut or punched to a desired size. During the drying step, the solute is removed and the dispersed particles coalesce to produce a continuous phase for suspending the photoconductor particles.

This second prior art process depends on maintaining a suitable dispersion of normally incompatible substances. This is difficult, particularly in the presence of high pigment concentrations such as used in the usual coatings for electrostatic printing.

The foregoing prior art processes produce electrophotographic recording elements which may be used to produce single copies of a suitable original or which may be converted into lithographic printing plates for making a plurality of duplicate copies by chemical treatment after a powder image of hydrophobic material is fixed thereto.

2 Such chemical treatment imparts hydrophilic properties to the coating surface.

An object of this invention is to provide improved electrostatic printing processes and improved recording elements therefor.

A further object is to provide improved compositions for producing recording elements for electrostatic printmg.

Another object is to provide recording elements for electrostatic printing having a hydrophilic surface which is particularly adaptable for producing lithographic printing masters by an electrostatic printing process without further chemical processing.

Generally, the recording elements of the invention comprise a photoconducting layer including a major proportion of a finely-divided photoconductor, such as a photoconductive zinc oxide, dispersed in a minor proportion of a water-soluble, electrically-insulating, film-forming vehicle such as polyvinyl alcohol. The photoconducting layer may be self-supporting but is preferably supported on a backing such as paper. The photoconducting layer may include also a bivalent or multivalent salt to make the layer particularly useful in preparing lithographic printing plates without further chemical processing. The

' Vehicle of the recording element is preferably insolubilized, as by polymerization, prior to use in electrostatic printing.

The invention further includes electrostatic printing processes comprising the steps of producing an electrostatic charge image corresponding to an applied light image, developing said electrostatic image with a finelydivided developer substance and then fixing said developed image substantially in situ. Optionally, the photoconducting coating may be dried inunediately before the step of charging.

The foregoing objects and other advantages will be more fully described in the following detailed description when read in conjunction with the accompanying drawings of which:

FIGURE 1 is a partially-schematic, sectional view of an apparatus for producing a blanket electrostatic charge upon a recording element produced by the method of the invention.

FIGURE 2 is a partially-sectional, elevational view of an apparatus for projecting a light image upon the charged recording element of FIGURE 1, and

FIGURE 3 is a sectional view of an apparatus for developing an electrostatic image upon the recording element of FIGURE 2.

Similar reference characters are applied to similar elements throughout the drawings.

A detailed example of preparing an electrophotographic recording element according to the invention will now be given.

Example 1.--Prepare a mixture of the following composition:

Polyvinyl alcohol g 5 Methanol ml 60 Distilled water ml 40 GP. photoconductive zinc oxide g The polyvinyl alcohol is dissolved in the water-methanol mixture, and the zinc oxide dispersed therein as by blending in for 5 minutes in a high speed blendor. The blended mixture is coated on a paper backing to a thickness to provide a final dry coating of about 0.0005 inch. The coating is thoroughly dried and then cut or punched to the desired shape and size. The product is an electrophotographic recording element comprising a paper backing having a white, matte, photoconducting coating thereon, which element is especially adapted for use in electro static printing.

While the backing of the example is paper, any substrate may be used. Thus, one may use cellulosic material such as cellophane or cellulose acetate, a metallic material such as copper, aluminum or brass, or a mineral material such as glass or mica. The substrate or backing may be in any desired shape or configuration. It is preferred, although not necessary, that the substrate have a higher electrical conductivity than the final photoconducting coating.

The coating applied to the backing will determine the spectral response, the speed of response and the contrast characteristic of the printing base. By a proper choice of the photoconductor and the vehicle, almost any spectral response, speed of response or contrast characteristic may be obtained within wide ranges. Almost any powdered pho-tocondnctor having sufliciently high value of surface photoconductivity may be used in the coating, for example, the photoconductive oxides, sulphides, selenides, tellurides, and iodides of cadmium, mercury, antimony, bismuth, thallium, molybdenum, aluminum, lead or zinc. In addition arsenic trisulphide, cadmium arsenide, lead chromate or selenium may be used. It is preferable for the photoconductor to have a high electrical resistivity in the darkness. Mixtures of one or more photoconductors may be used.

The particular photoconductor utilized determines the spectral response of the recording element. The color of the photoconductor indicates approximately the location or" the absorption edge of the photoconductor and of the recording element. Most photoconductors absorb light in the shorter wavelengths. When longer wavelengths are used, a value is reached where the absorption drops off sharply and the photoconductor ceases to absorb radiation. This value is called the absorption edge of the material. It is of particular advantage that by making a proper selection of the photoconductor that one may obtain a printing base with any desired light absorption characteristic and thereby desired spectral sensitivity. For example, thallium iodide has a peak response around 4130 A. Silver sulphide has a peak response around 13500 A., while other photoconductors may have their peak responses at other wavelengths in the electromagnetic spectrum and over a narrow or wide band of frequencies.

The electrically-insulating, film-forming vehicle is an essential part of the composition and may be any one of a number of substances which form dispersions in aqueous media. Most desirable is a vehicle having a high dielectric constant and high dielectric strength. These materials may be any water soluble natural or synthetic resin or gum, for example polyvinyl alcohol, hydroxyethyl cellulose and carboxymethyl cellulose, gum arabic or guar gum.

-A plasticizer may be used in conjunction with the vehicle to impart flexibility and pliability to the final coating. The choice of the plasticizer is determined by the vehicle used. The quantity of plasticizer used is determined by the degree of flexibility required in the coating and by the particular polymer used as the vehicle. This quantity may be present in an amount between zero and eighty percent by weight of the vehicle.

The proportion of powdered photoconductor to vehicle in the final coating may vary over a very wide range although the photoconductor should comprise a major proportion of the composition. The preferred ranges are 100 to 900 parts by weight of photoconductor to 100 parts by weight of vehicle. The optimum proportion will depend upon the nature of the photoconductor, the nature of the vehicle and the results desired.

The speed of response of the printing base particularly depends upon the nature of the photocondnctive material, the nature of the vehicle and the ratio by weight of photoconductor to. vehicle. Since the speed of response depends upon a number of characteristics, almost any de- 4 sired response may be obtained by the proper selection of materials and composition.

A proper selection of materials and compositions will also determine how long an electrostatic image may be stored on the surface of the photoconductive coating since storage of the electrostatic image depends upon the electrical resistivity of the material. Generally, the higher the resistivity of the coating the longer the storage time for the material.

The mixtures of the invention may be coated on the backing by any convenient method. For example, the coating may be sprayed on or flowed on or the backing may be dipped into the mixtures. Following the coating step, the coating is dried. The drying temperature is not critical. 'It should be sufiiciently high to evaporate the water but not so high as to burn or char the backing or constituents in the coating.

The coating of the final product may be any thickness. However, it is preferred that the coating thickness be about 0.0003 to about 0.002 inch. For the coating of the example, a coating 0.0005 inch thick is preferred.

A dye for sensitizing the photoconductive coating may be incorporated into the coating when the dispersions are prepared or after the coatings have dried.

Referring to FIGURE 1, the recording element of Example 1, which comprises a backing 21 having a. photoconducting coating 23, may be utilized in an electrostatic printing process according to the following steps. If the recording element is stored at low humidity it may be used directly. Otherwise, it may be desirable to dry the photoconducting coating as by heating or dessicating for a short time. The recording element is placed with the backing 21 upon a grounded metal plate 25 and an electrostatic charging device 61 passed in darkness over the photoconducting coating 23 to provide an electrostatic charge thereon. The charging device 61 may comprise an array of fine wires 53 mounted near the grounded metal plate 25. A source of DC. high voltage is connected between the wires 53 and the grounded plate 25 to provide a negative charge on the wires with respect to the grounded plate 25. The voltage should be sufficiently high to cause a corona discharge adjacent the wires. The recording element passing under the charging device 61 becomes charged negatively. The apparatus and process may produce a blanket positive charge if the polarity of the wires 53 is positive with respect to the grounded plate 25.

The next step in the processing is to discharge selected parts of the charged surface of the printing base in order to produce an electrostatic image thereon. Referring to FIGURE 2, this may be accomplished by exposing the printing base to an optical image derived, for example, from a projector 59 containing a photographic transparency of the subject matter to be printed. The optical image is focused on the charged surface of the photoconductive coating 23. The subject matter to be printed may, however, be any subject used in ordinary photographic processes. Any type of electromagnetic radiation may be used depending on the spectral sensitivity of the photoconductive coating 23. For example, visible light, infra red, ultra violet, and X-rays may be used.

Wherever the light strikes the surface of the photoconductive coating '23, the electrostatic charge thereon is reduced or removed. This leaves an electrostatic image or pattern of charges corresponding to the dark portions of the light image. Other methods of producing an electrostatic image may also be used.

The electrostatic image may be stored for a time if desired. Ordinarily the next step is to develop the electrostatic image with a finely-divided developer substance such as a finely-divided powder or an ink mist. Referring to FIGURE 3, development of the electrostatic image is preferably accomplished by maintaining the recording element in darkness and passing a developer brush 55 containing a developer powder across the surface of photoconductive coating 23 bearing the electrostatic image. Areas of developer powder 27 are deposited on those areas of the surface retaining an electrostatic charge. The developer brush comprises a mixture of magnetic carrier particles, for example, powdered iron and the developer powder. The mixture is secured in a magnetic field by a magnet 57 to form a developer brush.

A preferred carrier material for the developer mix consists of alcoholized iron, that is, iron patricles free from grease and other impurities soluble in alcohol. These iron particles are preferably relatively small in size, being in their largest dimension about .002 to .008". Satisfactory results are also obtained using a carrier consisting of iron particles of a somewhat wider range of sizes up to about .001 to .020".

A preferred developer powder may be prepared as follows. A mixture comprising 200 grams of 200 mesh Piccolastic resin 4358 (an elastic thermoplastic resin composed of polymers of styrene, substituted styrene and its homologs) marketed by the Pennsylvania Industrial Company, Clairton, Pa., 12 grams of Carbon Black G, marketed by the Eimer and Amend Co., New York, N.Y., 12 grams of spirit Nigrosine S.S.B., marketed by the Allied Chemical and Dye Co., New York, N.Y., and 8 grams of Iosol Black, marketed by the Allied Chemical and Dye Co., New York, N.Y., are thoroughly mixed in a stainless steel beaker at about 200 C. The mixing and heating should be done in as short a time as possible. The melt is poured onto a brass tray and allowed to cool and harden. The hardened mix is then broken up and ball-milled for about 20 hours. The powder is screened through a 200 mesh screen and is then ready for use as a developer powder. This powder takes on a positive electrostatic charge when mixed with glass beads or iron powder. It therefore develops an electrostatic image composed of negative charges. Two to four grams of the developer powder and 100 grams of the magnetic carrier material are blended together giving the completed developer mix. Other ratios may be used.

The developer powder may be chosen from a large class of materials. The developer powder is preferably electrically charged to aid in the development of the electrostatic latent image. The powder may be electrically charged because the powder (1) is electroscopic, or (2) has interacted with other particles with which it is triboelectrically active or (3) has been charged from an electric source such as a corona discharge. Examples of suitable developer powders are powdered zinc, powdered copper, carbon, sulphur, natural and synthetic resins or mixtures thereof.

The developer powder may be applied to the image in other ways, for example, it may be dusted on to the image, or it may be mixed with glass beads or other suitable carrier particles and then bringing the mixture into contact with the surface of the printing base. The beads serve merely as a temporary carrier, releasing the powder particles upon contact with the charged surface.

The type of powder described is a positively-charged powder and will adhere readily to negatively charged areas of the electrostatic image. In the developed image described, the developed areas of the image correspond to the non-illuminated portions of the optical image. If the printing base is charged positively, and the same steps are carried through as above described, a reverse image is obtained. If a negatively charged powder is used in place of the positively charged powder, then a reverse image is obtained in the alternative case.

The developed image 27 is now fixed to the photoconducting coating 23. If the developer powder or vehicle in the photoconducing coating 23 has a relatively low melting point, the image may be fixed by heating, for example, with an infra red lamp to fuse the powder to the surface. The powder image is preferably fused through the photoconductive coating 23. Sulphur or synthetic resin powders may be fixed in this way. Alternatively, the powder image 27 may be pressed into the coating 23. Another method of fixing the powder image 27 is to apply a thin coating of a solvent for the material of the powder image 27. The solvent may soften the developer powder particles and cause them to adhere to one another and to the coating 23. Alternatively, a solvent may be used to soften the photoconducting coating 23 and cause the developer powder particles to adhere thereto. Upon standing and preferably with the application of a slight amount of heat the solvent is evaporated from the printing base.

Example 2.Another formulation according to the invention is as follows:

The dimethylolurea acts as a polymerizing agent for the polyvinyl alcohol, rendering the dried coating water insoluble upon aging. The ammonium nitrate acts as a polymerization catalyst. Addition of water-soluble compounds such as sodium methyl siliconate (available from the Silicone Products Division, General Electric Co.) which impart a hydrophobic character to the dried surface, assist in preserving good charge storage properties in high humidity environments.

The mixture of Example 2 is coated on a backing and dried as described for the coating of Example 1. It may then be used in electrophotography as described in the foregoing description.

Example 3.--Another formulation according to the invention which is particularly useful in producing lithographic printing masters is as follows:

The following mixture is prepared- The zinc acetate functions to render the photoconducting coating receptive to water after the image is fixed to the coating. In the place of the zinc acetate in the above formulation, other water soluble bivalent metal salts may be used, for example, the water soluble acetates, nitrates, chlorides and formates of barium, cadmium, calcium, cobalt, copper, iron, lead, magnesium, nickel, strontium and zinc may be used. Ordinarily salts are used in concentrations between 5 and 15% by weight, however, in some cases they may be used in concentrations as high as 25% by weight. All the water soluble salts of bivalent metals selected from groups 1, 2, 4, 7 and 8 from the periodic table which are sufficiently stable in water solution may be used. In addition to the bivalent salts, water soluble multivalent metal formate or acetate when used in conjunction with a water soluble alkaline metal or ammonium formate or acetate may be used.

The developer powder used to prepare lithographic printing plates is a hydrophobic material such as the preferred developer powder described above. Upon fixing the powder image to the photoconducting coating, the recording element may be used for lithographic printing without further treatment. The recording element is swabbed with a wetting solution such as Platex and then printing in conjunction with a fountain solution such as Repelex in normal dilution. Platex and Repelex (trademarks), are marketed by the Addressograph-Multigraph Corporation, Cleveland, Ohio.

Example 4.-Another formulation according to the invention is as follows:

The following mixture is prepared Methyl cellulose (water-soluble grade) g.... Distilled water ml 200 20% zinc acetate in distilled water ml 75 dimethylolurea in distilled water ml 20 Photoconducting zinc oxide g 100 Methyl alcohol (to aid drying) ml 100 Example 5.Another formulation according to the invention is as follows:

The following mixture is prepared- Guar gum g 5 Distilled water ml 200 20% zinc acetate in distilled water ml 75 10% dimethylolurea in distilled water ml 20 Photoconducting zinc oxide g 100 Methyl alcohol (to aid drying) ml 100 Example 6.--Another formulation according to the invention is as follows:

The following mixture is prepared- Gum arabic g- 5' Distilled water ml 200 20% zinc acetate in distilled water ml 75 10% dimethylolurea in distilled water ..ml 20 Photoconducting zinc oxide g 100 Methyl alcohol (to aid drying) m1 100 There have been described compositions and methods for using these compositions to produce electrophotographic recording elements for electrostatic printing. The compositions are simple and economical to prepare and use, do not present a fire or a health hazard of organic or highly volatile solvent coatings and may be used in continuous web coating equipment designed for silver halide emulsions. There have also been described improved electrostatic printing processes including the electrophotographic recording elements of the invention.

What is claimed is:

1. An article of manufacture adapted for use in elec- 8 trostatic printing comprising a backing sheet coated with a photoconducting composition including about 100 to 900 parts by weight of photoconducting zinc oxide and about 100 parts by weight of a vehicle consisting essentially of about 5% to 25% by weight of zinc acetate and about 95% to about by weight of resinous polyvinyl alcohol.

2. A photoconducting composition for use in electrostatic printing comprising about 100 to 900 parts by weight of photoconducting zinc oxide and about 100 parts by weight of a vehicle consisting essentially of about 5% to 25 by weight of Zinc acetate and about to 75 by weight of resinous polyvinyl alcohol.

References Cited in the file of this patent UNITED STATES PATENTS 1,635,110 Bristow July 5, 1927 1,700,404 Goodell et al. Jan. 29, 1929 2,236,061 IZard Mar. 25, 1941 2,251,296 Shipp Aug. 5, 1941 2,287,161 Ball June 23, 1942 2,302,816 Toland et al. Nov. 24, 1942 2,367,420 Mullen Jan. 16, 1945 2,476,800 Blackburn July 19, 1949 2,534,650 Worthen Dec. 19, 1950 2,543,801 Patterson Mar. 6, 1951 2,662,832 Middleton Dec. 15, 1953 2,663,636 Middleton Dec. 22, 1953 2,735,785 Greig Feb. 21, 1956 2,875,054 Griggs Feb. 24, 1959 OTHER REFERENCES Elyanol Polyvinyl Alcohol, E. I. du Pont de Nemours, Electrochemical Department, Vinyl Products Division (1953), pp. 5-43.

The Merck Index, 6th Ed., Merck (1952), p. 579, Lithopone.

Wainer: Photographic Engineering, vol. 3, No. 1, pp. 12-22 (1952).

Young et al.: RCA Review (December 1954), pp. 469-484. 

1. AN ARTICLE OF MANUFACTURE ADAPTED FOR USE IN ELECTROSTATIC PRINTING COMPRISING A BACKING SHEET COATED WITH A PHOTOCONDUCTING COMPOSITION INCLUDING ABOUT 100 TO 900 PARTS BY WEIGHT OF PHOTOCONDUCTING ZINC OXIDE AND ABOUT 100 PARTS BY WEIGHT OF A VEHICLE CONSISTING ESSENTIALLY OF ABOUT 5% TO 25% BY WEIGHT OF ZINC ACETATE AND ABOUT 95% TO ABOUT 75% BY WEIGHT OF RESINOUS POLYVINYL ALCOHOL. 